This invention relates to a sewing machine in general and more particularly to a control means used in combination with an electronically controlled sewing machine for controlling an amount of a lower thread to be supplied in each stitch forming operation.
With the conventional sewing machines, a stitch is formed on a fabric by interlocking an upper thread carried by a vertically reciprocating needle and a lower thread supplied from a lower thread bobbin rotatably contained in a bobbin case which is, in turn, contained at a standstill in a rotating loop-taker. The loop-taker is mounted below a needle plate having a needle penetrating hole and operated in synchronism with reciprocation of the needle to interlock the upper thread with the lower thread in a well known manner.
There has been proposed many types of lower thread supplying mechanisms. An amount of the lower thread to be supplied from the bobbin for producing each stitch is determined theoretically based on the stitch control data of the pattern to be produced, which are stored in a pattern memory and read out therefrom to be supplied to a microcomputer mounted in the sewing machine. The microcomputer processes the stitch control data to determine coordinates of each needle dropping point. The lower thread supplying amount will therefore be theoretically determined in accordance with a distance between the one and the next needle dropping points located at different coordinates which have thus been determined by the stitch control data. In some cases, a thickness of a fabric on which the pattern is to be produced is detected by a sensor means, so that the fabric thickness is also taken into consideration in determination of the lower thread supplying amount.
In actual sewing machine operation, however, it has often been found that the lower thread supplying amount which will be determined on a theoretical basis in the above-described manner would not reflect an amount of the lower thread to be actually required for producing a stitch. Such discrepancy may be caused by various fluctuation factors, including a tension of the upper thread used in combination with the lower thread, friction properties of the upper and lower threads, and a load of the lower thread supplying mechanism. If an optimum amount of the lower thread should not be supplied, there would result in a deformed stitch or other stitching troubles.